2051
radiation (200-W medium-pressure lamp) of a 0.15 M solution of 2a in hexane for 6 hr produced a photo0
H la, Y = H b,Y P NO2
X
X
\
hu
#
Reaction of 0-Methylbenzohydroximoyl Chlorides with Sodium Methoxide. Inversion of Configuration at Trigonal Carbon during Nucleophilic Substitution
Sir : The reaction of phosphorus pentachloride with alkyl benzohydroxamates provides a general preparative method for 0-alkylbenzohydroximoyl chlorides. l t 2 This previous work' indicated that only one geometric isomer of a hydroximoyl chloride is formed in the reaction. In order to study the stereochemistry of the nucleophilic substitution reactions of these compounds, we have prepared and identified the E and Z isomers of 0-methylbenzohydroximoyl chloride (2a and 3a) and 0-methyl-p-nitrobenzohydroximoyl chloride (2b and 3b). The reaction of phosphorus pentachloride with methyl benzohydroxamate (la) gave 2a (71%) as a colorless oil, bp 53-54' (0.1 mm).3 Ultraviolet ir(1) J. E. Johnson, J. R. Springfield, J. S . Hwang, L. J. Hayes, W. C. Cunningham, and D . C. McClaugherty, J. Org. Chem., 36, 284 (1971). (2) For recent reviews concerning imidoyl chlorides, see (a) H. Ulrich, "The Chemistry of Imidoyl Halides," Plenum Press, New York, N. Y., 1968; (b) R. Bonnett in "The Chemistry of the Carbon-Nitrogen Double Bond," S . Patai, Ed., Interscience, New York, N. Y., 1970, pp 628-642. (3) All new compounds reported herein gave spectral and elemental analyses in accord with the assigned structures.
OCH3
@c=N
Y'
sa, X = C1; b, X = C1;
2a,X C1; Y H b, X = C1; Y =NO2 c, X =
Y =H Y =NO2
Br;Y =H
stationary state of the isomers 2a and 3a in a 62:38 ratio. The mixture was separated by preparative glc and isomer 3a was obtained as a colorless oil, bp 43-44' (0.1 mm). The geometric isomers of O-methyl-pnitrobenzohydroximoyl chloride (2b, mp 121-1 22' and 3b, mp 110-112") were prepared in a similar manner. Stereochemical assignments for the hydroximoyl chlorides were made on the basis of the dipole moment data given in Table I. Table I. Dipole Moment Data for E and Z Isomers of 0-Methylbenzohydroximoyl Chloride and 0-Methyl-p-nitrobenzohydroximoyl Chloride
(21) Alfred P. Sloan Foundation Fellow.
Niels H. Andersen,*Z1 C. Richard Costin Daniel D. Syrdal, Dana P . Svedberg Department of Chemistry, University of Washington Seattle, Washington 98195 Received October 27, 1972
/
d=N'"cH3 - -
Y# I I1 111 These limited (by space) examples should serve to illustrate the ABP model and its relationship to the other, apparently conflicting, viewpoints on this topic. An examination of the effects of specific vinylic and allylic substitution in rigid systems is in progress in order to delineate the relationship between ABP and torsional contributions. Acknowledgments. N. H. A. wishes to acknowledge stimulating conversations with Professors Scott, FCtizon, and Burgstahler on this subject. We are grateful to Professors FCtizon and Scott for sending preprints and unpublished results. The work was partially supported by National Institutes of Health Grant GM18143.
\
Hydroximoyl chloride 2a 2a 3a 3a 2b 2b 3b 3b
Conformation s-trans s-cis s-trans s-cis s-trans s-cis
s-trans s-cis
Theor dipole moment, Da 1.13 2.46 2.00 1.65 3.55 1.76 2.98 2.78
Exptl dipole moment, D ' 1.38(1.41)" 1.99 3.60 2.96 (3.00p
Calculated using the following bond (or group) moments and bond angles: CHI-0, 1.06 D ; N-0, 0.30 D ; C=N, 1.40 D ; C-C1, 1.75 D ; NO2, 4.20 D ; 110" for N-O-CHI; 120" for EN-0; 120" for N=C-C1. * T h e experimental dipole moments were determined in benzene at 25" using the method of E. A. Guggenheim, Trans. Faraday SOC.,45, 714 (1949); J. W. Smith, "Electric Dipole Moments," Butterworths, London, 1955, pp 58-64. Duplicate determination.
We have investigated the stereochemistry of the reaction of sodium methoxide with the hydroximoyl chlorides 2 and 3 in 90% dimethyl sulfoxide (DMSO)10% methanol. Reaction of a 0.07 M solution of 2a at 45" with sodium methoxide (0.09 M ) resulted in almost exclusive formation of a compound with inverted configuration, methyl (Z)-0-methylbenzohydroximate (4a, 68% yield). Only about 2% of the product could be attributed to the E isomer (5a) by glc. Under identical reaction conditions the (Z)-hydroximoyl chloride (3a) gave predominantly the product with inverted configuration, 5a (ratio of 4a to 5a = 23 :77 by glc, 62% yield). In order to establish the configuration of 4a and 5a, they were independently synthesized by methylation of the 2 and E isomers of methyl benzoCommunications to the Editor
2052 Y
CH,,O,
CH .O/c=K\ 4a, Y
=H
b. Y = NO,
OW,
Y
6= "OCHJ
5a, Y = H
wise) t o give a tetrahedral intermediate which undergoes a methoxide induced trans elimination of hydrogen chloride (as illustrated for 2a and 3a).6 In order for this mechanism t o be valid, it must be assumed that nitrogen inversion is slower than rotation about the carbon-nitrogen single bond in the tetrahedral intermediate. This assumption seems reasonable since 2a
b, Y =NO,
hydroximate whose configurations are known. Comparable results were obtained when the p-nitrohydroximoyl Chlorides, 2b and 3b, were allowed to react with sodium methoxide. Compound 2b produced 4b (4b :5b = 98 : 2) whereas 3b gave mainly 5b (4b : 5b = 16 : 84). Further investigation of the (Z)-hydroximoyl chloride (3a) reaction revealed that the lower stereoselectivity of this reaction is due in part to a methoxide induced isomerization of 3a to 2a. The hydroximates 4a and 5a were found to be stable under the reaction conditions; no isomerization of these compounds could be detected by glc over a 48-hr period. When the reaction of 3a with methoxide was quenched with water after approximately one half-life, it was found by glc analysis that some isomerization of 3a to 2a had occurred (ratio of 3a to 2a after approximately one half-life = 87 :13). That this isomerization is catalyzed by methoxide ion was demonstrated by the fact that an 0.08 M solution of 3a in DMSO-methanol did not undergo any detectable isomerization at 44.6" over a 48-hr period. The rates of the hydroximoyl chloride-sodium methoxide reactions were followed by quenching the reactions with water and titrating. Both 2a and 3a follow second-order kinetics through at least 60-70 of the reaction. The rate constant for the reaction of 2a (8.02 X lo-* M ) with sodium methoxide (2.88 X lo-? M ) was found to be 1.27 X l./(mol sec) at 44.6". At the same temperature, 3a (7.78 X M) reacted with sodium methoxide (2.67 X l e 2 M ) at a rate of 1.44 X lo-' l./(mol sec). In order to determine the effect of the leaving group on the rate of this reaction, 0-niethylbenzohydroximoyl bromide (2c, assumed to have the E configuration) was synthesized in low yield by the reaction of l a with phosphorus tribromide. The second-order rate constant for the reaction of 2c (8.06 X lo-* M ) with sodium methoxide (2.47 X M ) is only 1.9 times faster than that of the (E)-hydroximoyl chloride (2a)-sodium methoxide reaction. Stereoselective inversion of configuration during nucleophilic substitution at sp* hybridized carbon has not been reported heret0fore.j Although additional work is required for a detailed mechanistic proposal, our preliminary results are consistent with the premise that these reactions proceed by a methoxide-catalyzed trans addition of methanol (either concerted or step(4) ( a ) A. W m i c r and J. Suback, Ber., 29, 1153 (1896); (b) 0. Exner, V. Jehlicka, and A . Reiser, Collect. Czech. Chem. Commun., 24, 3207 (1959); (c) R. Blaser, P. Imfeld,and 0. Schindler, Helo. Chim. Acta, 52, 569 (1969). ( 5 ) All stereoselective substitution reactions at vinylic carbon proceed with retention of configuration, G. Modena, Accounts Chem. Res., 4, 73 (1971). Since submission of this work three reports on inversion during solvolysis of vinyl trifluoromethanesulfonates have appeared: (a) T. C. Clarke, 0. R. Kelsey, and R. G. Bergman, J . Amer. Chem. Soc., 94, 3626 (1972); (b) T. C. Clarke and R. G. Bergman, ibid., 94, 3627 (1972); (c) R. H. Summerville and P. v. R. Schleyer, ibid., 94, 3629 (1972).
Journal of the American Chemical Society
It OCH ,
-
C1
-HCI
rotation
4a
OCHj
H
H 6 nitrogen inversion
t
OCH,,
c1
H
H
7
-+ C H , O H
1I 3a
recent work indicates that nitrogen inversion is slow in N-benzyl-O,N-dimethyIhydro~ylamine.~ It is conceivable that the lower stereoselectivity of the 3a reaction is due to some inversion at nitrogen in the tetrahedral intermediate 7 to give its diastereoisomer 6.* Inversion at nitrogen could account for the partial stereomutation of 3a as well as the formation of 4a during the course of the reaction. Acknowledgments. We are indebted to the Robert A. Welch Foundation for financial support. The authors thank Dr. Timothy Rose, Texas A & M University, for making arrangements for us to use a Kahl dipolmeter. (6) The relatively small rate increase observed for tho reaction of 2c with sodium methoxide would appear to rule out an S N type ~ substitution mechanism. (7) D. L. Griffith, B. L. Olson and I. D. Roberts. J . Amer. Chem. Soc., 93, 1648 (1971), and references cited therein; M. Raban and D. Kost, J . Org. Chem., 37, 499 (1972). (8) Since 6 and 7 are diastereoisomers, it is reasonable to assume that the equilibrium between 6 and 7 lies i n favor of 6 . It is not clear why 6 is more stable than 7.
James E. Johnson,* Elizabeth Ann Nalley Department of Chemistry, The Texas Womutz's University Denton, Texas 76204 Charles Weidig Department of Chemistry, Sam Houston State Utiiversiiy Huntsville, Texas 77340 Receiced March 6 , I972
Rate of Intersystem Crossing between 'A and of an Iron(I1) Complex in Solution
States
Sir : Octahedral complexes of transition metals possessing four to seven d electrons can exist in either high-spin
1 95.5 1 March 21, 1973
